1. Field of the Invention
This invention relates to a catadioptric optical system suitable for projecting and transferring a circuit pattern on a mask onto a photosensitive substrate.
2. Related Background Art
An optical system suitable for projecting and exposing a mask pattern onto photoresist on a wafer when manufacturing an integrated circuit such as LSI is disclosed, for example, in U.S. Pat. No. 4,812,028. The optical system of FIG. 8 of this U.S. Pat. No. 4,812,028, as shown in FIG. 12 of the accompanying drawings (but with different reference characters), is designed to have a first partial optical system K1 having reflecting surfaces M1-M3 and for forming the primary image I1 of an object O, and a second partial optical system K2 having a reflecting surface M4 and for forming a reduced image (secondary image) I2 by the light from the primary image I1.
In such a catadioptric optical system, it is necessary to provide an aperture stop for limiting the marginal portion of a beam of light passing through the optical path in the optical system for the setting of the numerical aperture of the optical system and the setting of the positions of the entrance pupil and exit pupil of the optical system. In the catadioptric optical system shown in FIG. 12, the design is such that the positions of the reflecting surfaces M2, M4 provide an aperture stop, that is, the principal ray intersects the optical axis Ax in the reflecting surfaces M2, M4.
However, the provision of an aperture stop at the positions of the reflecting surfaces M2, M4 has led to a problem that the degree of freedom of the optical design of the catadioptric optical system is restricted. For example, in maintaining telecentricity, refractive power arrangement is determined so as to set the position of the entrance pupil and the position of the exit pupil at infinity, but if the position of the aperture stop is predetermined as described above, it may become impossible to make such an optical design that refractive power arrangement is primarily determined to provide a good imaging performance.
It is also conceivable to provide an aperture stop at any other position than the reflecting surfaces, but in the catadioptric optical system, the optical path is turned back by the reflecting surfaces and therefore, beams of light often overlap each other at the position of the aperture stop. If, all marginal portions of a beam of light are limited by a single aperture stop, there will arise a problem that the other beam of light (the beam of light turned back by the reflecting surfaces) is intercepted.
Also, in the catadioptric optical system shown in FIG. 12, no consideration has been paid to the prevention of flare and ghost. This leads to a problem that flare and ghost occur on the image plane (secondary image plane), thus reducing the imaging performance.